1. Field of the Invention
The present invention relates to a power module including a power semiconductor element.
2. Description of the Background Art
The power semiconductor device is generally constructed to as a power module. The power module has a power semiconductor element, e.g. a power metal oxide semiconductor field effect transistor (hereinafter referred to as MOS FET), an Insulated Gate Bipolar Transistor (hereinafter called to as IGBT), or a diode, based on silicon. As shown in FIGS. 1A and 1B, a conventional power semiconductor device 50 includes a heat sink plate 65, and a ceramic plate 63 with a circuit pattern mounted thereon. Silicon-based power semiconductor elements 51, 52 are mounted on the ceramic plate 63, and are soldered or wired by aluminum wires 66, such that a circuit will be constructed. The power semiconductor elements 51, 52 are connected to external terminals, e.g. a collector terminal 53, an emitter terminal 54, a control signal terminal 72, and an emitter signal terminal 74. The power semiconductor device 50 has a case including the heat sink 65, a side plate case 60, a top cover 61, and gel 62 fills the case.
A silicon-carbide-based semiconductor element (hereinafter referred to as silicon carbide semiconductor element) is used practically instead of the silicon-based semiconductor element. The rated temperature of the silicon-carbide-based semiconductor element is over 150° C. In fact, the element is able to be operated up to about 300° C.
The elements of the power semiconductor device, e.g. an IGBT, a flywheel diode, are connected to the circuit pattern by soldering. Therefore, when the device is heated up to a temperature higher than 150° C., especially 300° C., the solder connecting between the elements is melted. On stopping the device, the solder solidifies. Therefore, if operating and stopping will repeat one after the other, connecting region will deteriorate because of repetition of melting and solidifying. On the other hand, the emitter electrode and the gate electrode are connected by aluminum wire. An excessive stress is generated on wire bonding because the temperature deference between on operating and on stopping may increase. Then, the connecting region will deteriorate. The temperature of the heat sink plate 65 become a temperature of 300° C. in the conventional package, so that the whole of the power semiconductor device has a high temperature. When the power semiconductor device is embedded in a inverter, the temperature of other circumference component of the inverter will be increased so that life of the inverter will be shortened.
A printed wiring board for power electronics is called as power board. When the temperature of a power semiconductor element mounted on the power board increases, the temperature of the power board will be increased. Since the power board has a weakness for high temperature, it is necessary that the temperature of the connection portion between the element and the printed wiring board should be avoid to become high. Further, heat conduction between the high temperature side and low temperature side should be suppressed.